High impedance band pass filter



- Jail 1940v v. D. LANDON 2,187,805

HIGH IMPEDANCE BAND r1155 FILTER Filed Nov. so, 1936 3nventorpractically low resistance and capacity.

Patented Jan. 23, 1940 ,,.2,1zrz,scs I HIGH IMPEDANCE BANDQPASS FILTERVernon D. Landon, Haddonfield, N J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application November 30,1936,Serial.No.113,4 60

I I 5 Claims.

- "The present invention relates to band pass filters of the highfrequency type, and has for its object to provide a .band pass filter ofthe socalled M-derived shunt type which may have high input and outputimpedance whereby-it is I adapted to provide coupling means between highA high gain amplifierrutilizing a crystal filter.

101 is shown in the patent to Mason 1,967,249v :of July 24, 1934. It isa further object of the present invention to provide a high frequencyamplifier of the type shown in Mason wherein the crystal I may be.eliminated without sacrificing desirable operation characteristics.- 1 zThe cost of providing-a satisfactory'crystal filter is relativelyhigh incomparison to thecost ofproviding tuned circuits and it is, therefore,

a still'further object of the present invention to provide as asubstitute for a crystal in a high frequency filter, a tuned circuitwhich may opband and at a much lower cost.

' the theoretical performance of an M-derived band pass filter withoutrequiring inductances of im- The invention will, however, be betterunder. 39 stood from the followingdescription when considered inconnection with the accompanying I drawing, and itsscope will be pointedout in the ppended claims.

In the drawing, v 1 is a h matic diagram f work embodying the'invention,

Fig. 2 is a schematic diagram of an equivalent filter network used inanalyzing the performance- 5 5; 1- is of theband pass, type adapted. toprovide A further object of the invention is to provide a, filter net-Referring to Fig. 1, a signal transmission cir' cuit represented byinput leads 5 are connected, through an inputcircuit impedanceB whichmay be the plate impedance of a preceding tube,- to a filter network Ihaving output leads 8. i The filter sharp attenuationzoneach side of thepass band and may be of the M-derived type with midshunt termination,asshownin Fig. 2 to which attention is now directed along with Fig. 1.

Broadly considered, the circuit of Fig. 1 comprisesa high impedanceinput terminal 9,,a high impedance output terminal l0 and a commonground terminal H with a T structure of capacitors C1, Cl-and C2:connected between the three terminals, the latter connection includingaresistor R2, hereinafter referred to.

The inductance elements of the filter network comprising inductances L1,L1 and L2 likewiseprovide a T structureconnected tothe same threeterminals and a resonant circuit 23 coupled to the two inductors formingthe arms of the T of inductors and aresistance R connected in the stemof the T of the inductors. I i

The filter of Fig. 1 embodies a circuit which is the equivalent inperformance to that shown in Fig."' 2. The same reference numerals andreference characters are used in both figures to designate like parts,and the circuit of Fig. 2 will be vrecognizedas a band pass filter inwhich the circuits comprising LzCz, L3C3 form the rejector elements ofthe filter. The circuit of Fig. 2 would be: ideal if reactors having theproperconstants could be designed. However, this can not practicallybedone, particularly in connection with the inductance L3, as designformulas call. for the inductance L3 to be very large to beresonated by'capacityCa which should be relatively low. 1

When an inductance of the proper value is. constructed for use at La, itis found to have a distributed capacity larger than the capacity calledfor at C3. Hence, it can not be resonated by C3. Also, thelosses in LsCsand L202 should be extremely small for best operation.

'Ihesedisadvantages in a circuit in accordance with. Fig.2 haveheretofore been overcome by utilizing a crystal in place of the circuitCsLz to provide sharp attenuation at each side of the pass band.However, in the circuit shown in Fig. l, the difficulties have also beenovercome in that the requirementfor a large inductance of unduly lowcapacity is eliminated by replacing L303 of Fig. 2 by a tuned circuitLsCsLs coupled to the input and output circuit of the filter. In thiscase the tuned circuit is coupled tothe input and output inductances L1and L1 of the filter as shown.

'Referrin'g'now to Figs. 3 to 6 inclusive, in addition to Figs. 1 and 2.and in which the same reference characters are used throughout, the

circuit of Fig. 2 can best be resolved into the circuit of Fig. 1 byprogressive steps illustrated by the figures referred to. For example,it will be seen that the circuit C3113 of Fig. 2 may be provided asshown in the circuit of Fig. 6, since the circuit of Fig. 6 may be shownto be equivalent to the circuit shown in Fig. 5, in which the seriesresonant circuit CsLx of Fig. 2 or of Fig. 5 is replaced by a circuitinductively coupled to the input and output inductances L1 and L1 ofFig. 6.

Similarly, L2C2 can be replaced by inductive and capacitive couplings asin Fig. 2. This is illustrated in Fig. 4 which can be shown to beequivalent to Fig. 3 by performing a T to 1r transformation inaccordance with the conversion formulas, examples of which are found inTransmission Circuits for Telephone Communications by K. S. Johnson,page 282, Fig. 27. The transformation is taken first on the T of theinductances and secondly on the T of the capacity.

If, as shown, Fig. 4 is equivalent to Fig. 3, and Fig. 5 is equivalentto Fig. 6, it follows that Fig. 1 is equivalent to Fig. 2. However, Fig.1 has the advantage that L3 and L2 may have more reasonable constants.

In Fig. 2 the reactances L2C2, L3--C3 are assumed to be dissipationless.If losses are present, the effect is chiefly to reduce the attenuationat the two frequencies adjacent to the pass band where the attenuationis desired to be infinite.

In the circuit of Fig. 1 the presence of losses in the circuit LsCa hasan effect analogous to losses in L303 in Fig. 2. However, resistance inseries with L2 appears mathematically as a negative resistance in serieswith the series arm of the filter when a T to 1r transformation isapplied to the T of inductances. Thus, while dissipation in LsCsdecreases the peak attenuation, dissipation in L2 or C2 increases theattenuation. A resistor may be inserted in series with L2 and adjustedto give infinite attenuation at the two attenuation peaks f1 and f2.This resistance may be inserted wholely in series with L2 in Fig. 1 asshown at R or may be included also in circuit with C2 as shown at R2.

The attenuation characteristic takes the form of the curve A of Fig. 8.The double attenuation peaks at frequencies f1 and f2 are provided bythe combination of series resonant and parallel resonant circuits, L2C2,L3C3 of Fig. 2, which are tuned to the same frequency f. The pass bandof the filter is effective between the frequencies is and f4.

It has been shown in the foregoing description that the circuit of Fig.l is equivalent to the circuit of Fig. 2. The latter is a well knownM-derived filter, as may be seen with reference to page 255, Fig. 141,of Transmission Networks and Wave Filters, by T. E. Shea, published byD. Van Nostrand Company in 1929.

The circuit parameters of Fig. 1 are obtained by the method used inshowing the equivalence, for example, of C1, C1, C2 of Fig. 1, which areobtained from the values of the same elements of Fig. 2 by well knownformulas relating to T and 1r circuits given in the work by Johnson,hereinbefore referred to.

The value of R in Fig. 1 to use for peak attenuation is, in general,found by experiment. However, the value is adjusted for peak attenuationat f1, f2. This value may be solved for by the following:

where R2 is the total resistance of the link circuit.

From the above reference in Johnson, the series arm of the Ir networkis, in Johnsons terminology:

(92L12 w L +R The non-reactive term is seen to be negative. If the bandis narrow so that (in Fig. 1) L1 is much greater than L2, then the lastterm of the above may be neglected for a first approximation. Then(except for the negative sign) the power factor of the series arm of the1r (L2 of Fig. 2) is the same as that of the stem of the T (L2 of Fig.1). If this power factor is the same as that of L3 C; then the branchcomprising L3 C3, L2 C2 (of Fig. 2) presents infinite impedance atfrequencies f1 and f2.

Then

In regard to the relation of the L1C1, L202, L302, Fig. 2, combinationsto the pass band and attenuation peaks, these may have various values,as discussed in Shea above referred to, where the circuit parameters areevaluated on Fig. 141 in terms of the equivalent constant K filter.

Referring to the circuit of Fig. '7, the use of the filter network as acoupling impedance between a high impedance high gain amplifier tube !2and a second amplifier tube 13 is shown. The ground lead l4 forms thecommon ground lead for the amplifiers, and the output circuit I5 oftheamplifler I2 is coupled to the impedance coupling means I8, I I withthe high impedance input terminal I 8 of the filter network 19. The highpotential output terminal of the filter network indicated at 2D isconnected through a similar impedance coupling means 2|, 22 with theinput electrode 23 of the amplifier I3. The signals-amplified by thedevice l2 are applied to the amplifier device l3 subject to the bandpass characteristic of the filter and to the attenuation characteristicthereof. This filter network corresponds in form to that shown in Fig. 2but indicates a practical coupling network for two amplifier tubesproviding high impedance output and input circuits which are to becoupled.

In form, the filter network shown is a 1r network of resonant elements,each element being a parallel resonant circuit tuned to the mean passband with a fourth circuit comprising the elements L3, C3, L3, alsoresonant to the mean pass band frequency and inductively coupled to thetwo shunt elements L1 and L1 of the 1r structure. The resistor R iscommon to the inductive branches L1 and L1 of the input and outputcircuits. Thus, as in Fig. 1, means are provided in the filter networkfor effecting the theoretical performance of a dissipationless M-derivedfilter.

In this circuit the 71' form of filter network is retained for thereactances L1L2L1 and the caatfthe'rejection frequencies.

pacity C1Ciz Ci, to show an operable variationof equivalent performanceto that of Fig. '1. As in Fig. 1 the resistance R'in series with L202 isgiven such a value that infinite attenuation is obtained LZ'V'YIIII theclrcuit 'of 2, if LcCa=LzCz=LiCn each pair of elements being resonant ata midband, frequency, atsome lower frequency LcCs being a series circuithas a capacitive reactance while L2C2, being a parallel circuit, has aninductive reactance. Thus, at a certain frequency below the pass band ofthe filter network the combination acts as a parallelresonant circuit. Asimilar action takes place above the transformation band and theimpedance developed by the combination of these two frequencies isinversely proportional to the resistance of the elements. This islikewise true of the circuits of Figs. 1 and 7. The attenuation peaks f1and f2, one on each side of the desired transmission band, are providedby the combination L3--C3 and L2C2, acting like a parallel resonantcircuit at each of the attenuation frequencies.

It will be seen that the difiiculty in the way of duplicating theperformance of an ideal filter because of the presence of resistance ineach element of. the circuit may be overcome by the corresponding filternetworks, as shown in Figs.

1 and 7, and may be rendered substantially as Fig. 2 is raised to aninfinitely high value, giving high attenuation at the proper points whenthe negative resistance is adjusted to balance out the other resistanceof the combination.

The circuit of Fig. 1 has been tested and found to give results whichare highly satisfactory as a substitute for a crystal filter. Theadvantages over the crystal filter are found in the low cost of theelements of the filter and flexibility in the design thereof.

An embodiment of the invention, as shown in Fig. 1, may have thefollowing constants:

f :175 kc. R :51 ohms f1:165 kc. L1=4300 microhenries f2=185 kc. L3=4300microhenries f3=170 kc. L2=940 microhenries f4=180 kc.

The capacitors are adjusted for resonance at 1'75 kc.

The terminal resonance for the above constants may be considered to be120,000 ohms. At f2 the best value of R was 62 ohms, and at f1 40 ohms.Therefore, a compromise value of 51 ohms was considered best to use. Thegain of the circuit when used between high impedance circuits, as inFig. 7, was approximately 50.

The resulting response curve was flat from 1'71 to 1'79 kc. and theattenuation on f2 and f1 was more than to 1, as indicated by the curvein Fig. 8.

The use of a tertiary circuit as in Fig. 1 has the advantage that thecost of the filter circuit may be reduced materially and permits morenearly the ideal circuit of Fig. 2 to be provided by reactance elementshaving practical physical and electrical characteristics.

'I claim as my. invention: 1. A band pass filter comprising, incombination, means providing input and output circuits therefor eachresonant to 'a mean pass band frequency'and including an inductanceelement,

means including additional inductance elements resistor in circuit-withthe inductance elements of the input and output circuits, the resistanceof the resistor being so proportioned with respect to the impedance ofsaid circuits that substantially infinite impedance is effected at twofrequencies adjacent to and on opposite sides of" resonance.

2. In a band pass filter,'the combination of a high signal potentialinput terminal, a high signal potential output terminal, a common groundterminal, a T structure of capacitors connected between said terminals,a T structure of inductances connected between said'terminals, theelectrical values of the elements of the two T- structures being suchthat an arm and the stem of the inductive T is resonant with an arm andthe stem of the capacitive T at the mean pass band frequency, meansproviding a resonant circuit inductively coupled to the arms of theinductance T-structure, and a resistance connected in the stem of saidlast named T structure having a value to provide substantially infiniteimpedance at rejection frequencies adjacent to and above and below themean pass band frequency.

3. A bandpass filter comprising, in combination a 1r.St1llCtllI'8 ofresonant elements, each element comprising a parallel resonant circuithaving inductive and capacitive branches and being tuned to a mean passband frequency, means providing a fourth circuit resonant to the meanpass band frequency and inductively coupled to the two shunt arms of the11' structure, and a resistor common to the inductive branches of saidshunt elements of the 1r structure, the resistance of the resistor beingso proportioned with respect to the impedance of said circuits thatsubstantially infinite impedance is effected at two frequencies adjacentto and on opposite sides of resonance.

4. In an electric discharge amplifier, the combination of meansproviding a high impedance output circuit and means providing a highimpedance input circuit, and a band pass filter comprising parallelresonant circuits connected in parallel relation to said output andinput .circuits, said parallel resonant circuits forming the arms of a1r structure and having inductive branches, a resistor common to theinductive branches of said parallel resonant circuits, a third parallelresonant'circuit forming the series arm of said 1r structure, and afourth circuit inductively coupled to the inductive branches of saidfirst named parallel resonant circuits, all of said circuits being tunedto a mean pass band frequency withinwhich the amplifier -operates.

5. A band pass filter providing substantially the performance of adissipationless M-derived filter comprising, in combination, a T networkof inductance elements and a T network of capacity elements providingtwo parallel T structures between high potential input and outputterminals for said filter and a common ground terminal, the electricalvalues of the elements of the two T networks being such that an arm andthe stem of the inductive T is resonant with an arm and the stem of thecapacitive T at the mean pass band frequency, means for increasing theresistance in the stem of at least one of said T structures to a valueto provide substan-

